EFFECT OF OSMOTIC SWELLING AND SHRINKAGE ON NA-K+ PUMP ACTIVITY IN MAMMALIAN CARDIAC MYOCYTES()

The effect on the sarcolemmal Na+-K+ pump of exposure to anisosmolar s olutions was examined using whole cell patch clamping and ion-selectiv e microelectrodes. Na+-K+ pump currents were measured in single ventri cular myocytes by using pipette Na+ concentrations ([Na]pip) of 0-70 m M. The relationship between [Na]pip and pump current was well describe d by the Hill equation. The [Na]pip for half-maximal pump current (K0. 5) was 21.4 mM in isosmolar (310 mosM) solution. K0.5 was 12.8 mM duri ng cell swelling in hyposmolar solution (240 mosM) and 39.0 mM during cell shrinkage in hyperosmolar solution (464 mosM). The maximal pump c urrents, derived from the best fit of the Hill equation, and the Hill coefficients were similar in isosmolar, hyposmolar, and hyperosmolar s olutions. A sustained (> 20 min) decrease in the intracellular Na+ act ivity developed during exposure of intact papillary muscles to hyposmo lar solutions, and a sustained increase developed during exposure to h yperosmolar solutions. We conclude that osmotic myocyte swelling stimu lates the sarcolemmal Na+-K+ pump at near-physiological levels of intr acellular Na+, whereas shrinkage inhibits the pump. These changes are due to increases and decreases, respectively, in the apparent affinity of the pump for Na+.